The present invention relates generally to medical imaging and, more particularly, to a system and method for acquiring and reconstructing magnetic resonance (MR) images having uniform fat suppression. Data acquisition is carried out using partial asymmetric acquisitions for dynamic contrast-enhanced imaging having uniform fat suppression.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
High spatial resolution contrast-enhanced 3D MR imaging techniques have been used clinically for evaluating particular regions-of-interest (ROI) or volumes-of-interest (VOI) to identify abnormalities and pathologies for clinical diagnosis. As is well known, these abnormalities or pathologies are commonly found in lesions in the subject. Moreover, these lesions are commonly found surrounded by fatty tissue. For example, liver metastases or breast lesions are often in close proximity to large concentrations of fat. To acquire MR data with contrast between the lesions and the surrounding fatty tissue, a fat saturation imaging technique is implemented to suppress the fat signal and, ultimately, to enhance the detectability of the lesions in the reconstructed image.
Typically, chemical shift preparation sequences, commonly referred to as “fat suppression,” are applied to an imaging space to suppress signals attributable to fat within an imaging subject which may otherwise interfere with identification of a lesion or other pathology. A number of imaging techniques have been developed to enhance contrast between fatty and non-fatty tissues. For example, fat saturation pulses have been shown to improve contrast. However, applying fat saturation pulses can extend the scan time and, therefore, unduly lengthen imaging processes, such as dynamic contrast enhanced imaging studies, as well as decrease patient throughput.
As such, techniques such as that illustrated in FIG. 1 have been developed to suppress fat while reducing scan times. FIG. 1 shows a traditional fast spoiled gradient recalled echo (FSGRE) sequence/data acquisition utilizing a spectrally selective inversion (SPECIAL) pulse 1 to suppress a signal from fat 2. The SPECIAL pulse 1 inverts only the fat magnetization (Mz) without affecting the water proton magnetization. The inverted fat magnetization 3 then recovers over time, across the axis when the fat magnetization or signal is zero 4, to steady state magnetization or full recovery 5. The point where fat magnetization is at zero is referred to as the null point for fat suppression 4. When the fat magnetization is nulled 4, MR data is acquired and used to fill k-space 6 in a centric fashion in the slice direction until all slice encoding Kz lines 7 are acquired. Therefore, for every phase encoding view, only one SPECIAL pulse 1 is applied followed by excitation pulses 8, also called αpulses, which encode all Kz lines 7. This technique is fast, however, it results in non-homogeneous fat suppression, ringing artifacts, and edge enhancement due to non-uniform fat suppression and excessive fat recovery. That is, the Kz lines 7 acquired at or near the null point 4 include little or no data from fat, while data acquired at or near the full fat recovery point 5 include a large influence from unsuppressed fat. Therefore, since data acquisition begins with full fat suppression and continues until fat magnetization has fully recovered 5, non-uniform fat suppression is included across the Kz lines 7, which results in ringing artifacts and edge enhancements in the reconstructed image.
Therefore, while the above-described imaging technique includes fat suppression and does not unduly extend scan times, non-homogeneous fat suppression and ringing may be observed within reconstructed images. That is, while using a spectrally-selective inversion pulse 1 with a centric encoding imaging sequence reduces scan times, it may also result in non-homogeneous fat saturation and is prone to the presence of ringing artifacts in reconstructed images.
It would, therefore, be desirable to have a system and method capable of uniform fat suppression without sacrificing image quality (IQ). Furthermore, such a system and method should be capable of completing an imaging scan within clinically acceptable limits for dynamic contrast enhanced studies.